Genetic analysis is widely used in basic and applied research as well as in diagnostics to screen, to profile and to genotype patients. Clinical laboratories currently offer genetic tests for more than 300 diseases or conditions including the analysis of mutations in the BRCA1 and BRCA2 genes, as well as in the p53, N-, C- and K-RAS, cytochrome P450, CFTR, HLA class I and II, Duchenne Muscular Dystrophy and beta-globin genes. The test menu continues to grow as advances in the Human Genome Project lead to the identification of genetic determinants that play a role in causing disease.
Genetic testing involves the analysis of genes and/or chromosomes to detect inheritable or other mutations as well as chromosome aberrations in order to provide a diagnosis for disease susceptibility. In addition, protein levels are monitored to obtain an indication of disease progression or response to treatment. Genetic testing has been used to diagnose and to monitor cancer, as well as to assess the pre-symptomatic risk of individuals to develop the disease. At present, for example, members of families diagnosed for several diseases such as Atexia-telangiectasia, Bloom's syndrome, Fanconi's anemia or Xeroderma Pigmentosum can be tested for the occurrence of mutations in the respective genes. In addition, several mutations in the regulatory gene p53 also have been correlated with the risk of developing different types of cancers. Those who inherit p53 mutations are at high risk of developing sarcoma, brain tumors or leukemia.
The standards analysis methods used in genetic analysis, DNA typing and DNA fingerprinting include (1) analysis of variable Number of Tandem Repeats (VNTR) (e.g., Nakamura et al., Science, Vol. 235, pp. 1616–1622 (1987), (2) analysis of Short Tandem Repeats (STR) (e.g., Edwards et al., Am. J. Hum. genet. Vol. 49, pp. 746–756 (1991); Ricciardone et al., Biotechniques, Vol. 23, pp. 742–747 (1997), (3) analysis of Single Nucleotide Polymorphisms (SNP) (e.g., Nickerson et al., Proc. Natl. Acad. Sci. U.S.A., Vol. 87, pp. 8923–8927 (1990); Nikiforov et al. Nucleic Acids Res. Vol. 22, pp. 4167–4175 (1994); Ross et al., Anal. Chem. Vol. 69, pp. 4197–4202)), (d) analysis of Restriction Fragment Length Polymorphisms (RFLPs) (e.g., Botstein et al. Am. J. Hum. Genet. Vol. 32, pp. 314–331 (1980)), and (4) analysis of mitochondrial DNA sequences. VNTR and STR analyses utilize simple or multiplex Polymerase Chain Reaction (PCR) technology (e.g., Mullis et al., Cold Spring Harbor Symp. Quant. Biol., Vol. 51, pp. 263–273 (1986); Mullis et al., Science, Vol. 239, Vol. 487–491 (1988)). RFLP analysis utilizes restriction enzyme digestion of DNA followed by DNA hybridization techniques with labeled probes; and mitochondrial DNA sequence analysis utilizes a combination of PCR technology and conventional dideoxy sequencing in a process known as cycle sequencing.
Variations among individuals in the number of STRs in specific genetic locations have been shown to be associated with several common genetic diseases. For example, unstable doublet repeats are known to be associated with disease states such as cystic fibrosis and colorectal cancer. Certain unstable triplet repeats are known to be associated with several genetic diseases, including Kennedy's disease, fragile-X syndrome and Myotonic dystrophy. Huntington's disease in particular has been investigated extensively and STRs have been mapped across a section of the gene to identify 51 triplet repeats spanning a 1.86 Mbp DNA segment. Higher-order repeats, such as tetramers, have also been associated with particular disease states including Huntington's disease and spinocerebellar ataxia type 1.
DNA typing based on the standard laboratory methods requires extensive sample preparation and significant post-PCR processing. The latter includes the steps of restriction enzyme digestion, agarose/acrylamide gel electrophoresis, sequence analysis or a combination of these methods. These multi-step protocols introduce considerable bias in the data and are labor intensive and time consuming.
DNA fingerprinting, also referred to as identity testing, relies on the analysis of highly polymorphic genetic loci to provide unambiguous molecular identification of individuals. A variety of polymorphic markers are available for this purpose including restriction fragment length polymorphisms (RFLPs), single nucleotide polymorphisms (SNPs), STRs/microsatellites and variable number of tandem repeats (VNTRs)/minisatellites. RFLP analysis requires enzyme digestion of genomic DNA followed by gel electrophoresis and hybridization of radiolabeled probes to the gel. The complexity of this procedure has prevented RFLP analysis from being widely adopted for identity testing. SNPs, wherein one allele differs from another allele at a single position, occur with an average frequency of 1 in 1,000 bases in both coding and non-coding regions and constitute 90% of all polymorphisms within the human genome (Brooker, Gene, 234:177–186 (1999)). They have been used for the mapping of genes associated with diseases such as cancer, for the typing of donors for bone marrow engraftment, and for studying inheritance within the context of population genetics.(Kwok at al., Mol. Med. Today, 538–543, (1999)) However, while suitable sets of SNPs are being developed to provide unambiguous DNA fingerprints, those new markers will require careful validation. In addition, in comparison to the STR markers commonly used at present, the set of SNP markers required to ensure a given probability of exclusion of ambiguity will be large. Both SNPs and STR polymorphisms can be used as markers, however about 7 to 12 SNPs per STR polymorphism are required to get a power of exclusion of 99.73%.
STRs and VNTRs are highly informative polymorphic markers. Many genetic loci contain a polymorphic STR region consisting of short, repetitive sequence elements, typically 3 to 7 bases in length. Trimeric and tetrameric STRs occur as frequently as once per 15,000 bases of a given sequence and are widely used for identity typing in parentage and forensic analysis. In contrast to the case for SNPs, where a large number of loci are needed for exclusion, only nine specific STR loci are required to provide a combined average power of exclusion of 99.73%.(Alford et. al Current Opinion in Biotechnology, 29–33 (1994), Latour et al, 829–37 (2001). STRs may be amplified via the polymerase chain reaction (PCR) by employing specific primer sequences directed to the regions flanking the tandem repeat.
Other polymorphisms arising from differences in the number of repeated elements in an allele include variable number of tandem repeats (VNTRs)/minisatellites, which are tandem repeats of a short sequence containing from 9 to 60 bases, and microsatellites which contain from one to five bases. Minisatellites and microsatellites are generally considered to be a subclass of VNTRs. Since it is estimated that about 500,000 microsatellite repeats are distributed throughout the human genome, at an average spacing of 7,000 bases, VNTR regions also can be used in identity testing.
In conventional laboratory practice, STRs and VNTRs are amplified by PCR using radio-labeled or fluorescence-labeled primers. The PCR products are separated by gel electrophoresis or capillary electrophoresis for identification.
In conventional implementations of genetic testing, information relating to sample and patient identification is recorded manually, typically involving the completion of bar coded labels which are affixed to sample collection containers. Such labeling procedures represent a potentially significant source of error involving mishandling, mislabeling and switching of samples.
Thus a need exists for a mechanism whereby collected known biological samples would be unambiguously marked and identified at the time of collection. This would safeguard against the mishandling, mislabeling and switching of samples during analysis.